This invention relates to light-emitting semiconductor devices, or light-emitting diodes (LEDs) according to common parlance, and particularly to those having provisions for preventing metal migration from the reflector layer incorporated in the LEDs. The invention also concerns a method of making such migration-proof LEDs.
LEDs in general have a semiconductor region composed and configured to generate narrow-spectrum light of a desired wavelength. Typically, the light-generating semiconductor region has an active layer sandwiched between a pair of confining layers or claddings of opposite conductivity types. Light is generated in the active layer when the device is electrically biased forwardly of the pn junction. Part of the light more or less directly traverses one of the claddings and issues from the light-emitting surface of the light-generating semiconductor region. The rest of the light is radiated toward the substrate via the other cladding. How to redirect the highest possible proportion of this light component back toward the light-emitting surface is one of the key factors that determine the efficiency of the LED.
It is itself not new in the art to provide a layer of reflective metal (hereinafter referred to as the reflector layer) between the light-generating semiconductor region and the substrate, as disclosed for example in Japanese Unexamined Patent Publication No. 2002-217450. The reflector layer suggested in this prior application is made from aluminum and interposed between the light-generating semiconductor region of Groups III-V compound semiconductors and the substrate of silicon. Other reflective materials are adoptable, though, such as silver or silver-base alloy.
The LEDs with such metal-made reflector layers, as so far constructed, possessed a shortcoming: The metal making up the reflector layer was easy of thermal migration onto the other parts of the LED either during or after the manufacture of the device. The migration was most easy to occur during LED manufacture when the light-generating semiconductor region with the reflector layer thereon, after having been grown on a substrate, was being united with a baseplate via a bonding metal layer or layers under heat and pressure. In use of the completed LED, too, the reflector metal would migrate as the device heated up by continued energization.
The likelihood of such reflector metal migration in use is more or less reducible by a protective film enveloping the sides of the LED at which are exposed the side edges of the reflector layer. However, should the protective envelope be not held fast enough against the side edges of the reflector layer by any chance, the reflector metal was easy to migrate onto the side surfaces of the light-generating semiconductor region, possibly short-circuiting the pair of claddings of opposite conductivity types across the active layer. The result was a significant drop in output light intensity.